<template>
  <ul class="list">
    <li class="item" 
        v-for="(item, index) of letters" 
        :key="item" 
        :ref="item"
        @click="handelLetterClick"
        @touchstart.prevent="handleTouchStart"
        @touchmove="handleTouchMove"
        @touchend="handleTouchEnd"> {{item}}</li>
  </ul>
</template>

<script>
export default {
  name: 'CityAlphabet',
  props: {
    cities: Object
  },
  computed: {
    letters () {
      const letters =[]
      for (let i in this.cities) {
        letters.push(i)
      }
      return letters
    }
  },
  data () {
    return {
      touchStatus: false,
      startY: 0,
      tiemr: null
    }
  },
  updated () {
    this.startY = this.$refs['A'][0].offsetTop //A元素距离顶部的高度
  },
  methods: {
    handelLetterClick (e) {
      this.$emit('change', e.target.innerText)
    },
    handleTouchStart () {
      this.touchStatus = true
    },
    handleTouchMove (e) {
      if (this.touchStatus) {
        if (this.tiemr) {
          clearTimeout(this.timer);
        }
        this.timer = setTimeout(() => {

          const touchY = e.touches[0].clientY - 79 //touches[0]手指的信息
          const index = Math.floor((touchY - this.startY) / 20)

          if (index >= 0 && index < this.letters.length) {
            this.$emit('change', this.letters[index])
          }        
        }, 16)        
      }
    },
    handleTouchEnd () {
      this.touchStatus = false
    }
  }
}
</script>

<style lang="stylus" scoped>
  @import '~styles/variables.styl'
  .list 
    position : absolute
    display : flex
    flex-direction : column
    justify-content : center
    top: 1.58rem
    right : 0
    bottom : 0
    width : .4rem
    .item 
      line-height : .4rem
      text-align : center
      color : $bgColor
</style>